1. Field of the Invention
The present invention relates to a developer for developing electrostatic latent images in electrophotography, electrostatic recording, electrostatic printing and the like.
2. Description of the Related Art
In developers used for developing electrostatic latent images formed on a latent image-bearing member in electrophotography, electrostatic recording, and electrostatic printing, storage characteristics (blocking resistance), transport characteristics, developing characteristics, transfer characteristics, charging characteristics, and fixing characteristics are particularly important. Heretofore, many means have been proposed for improving the above-mentioned characteristics. One such proposal has been a method wherein additives are added to the toner to improve flow characteristics and environmental resistance.
A common additive externally added to toner (hereinafter referred to as xe2x80x9cexterior additivexe2x80x9d) are silica microparticles and titania microparticles and the like. Conventionally, silica microparticles are used as a main exterior additive, used principally for maintaining flow characteristics. The silica microparticles are themselves easily affected by temperature and humidity, and are particularly subject to a large rise in charge under conditions of low humidity, which causes problems with the charge stability of the toner.
On the other hand, titania is not as readily affected by temperature and humidity compared to silica, but when titania microparticles are used as a main additive, the charge level of the toner is reduced due to the low charge level of the titania itself, thereby causing disadvantages of airborne dispersion and fog over long term use.
For these reasons silica microparticles are used to maintain toner flow characteristics and titania microparticles are added to minimize the adverse affects of temperature and humidity on the silica. The use of silica as a main exterior additive and titania as an adjunct exterior additive has been proposed (e.g., U.S. Pat. No. 4,904,558). Accordingly, when viewed from the overall developer, the developer characteristics largely depend on silica, and are easily affected by temperature and humidity.
Particularly when polyester resin is used as the toner binder resin, the polyester resin itself poses marked disadvantages in that it is readily influenced by temperature and humidity fluctuations due to the presence of hydrophilic groups such as ester bonds, hydroxyl groups, carboxyl groups and the like.
An object of the present invention is to eliminate the previously described disadvantages by providing a novel developer for developing electrostatic latent images.
Another object of the present invention is to provide a developer having stable charging and developing characteristics.
Still another object of the present invention is to provide a developer that is not susceptible to temperature or humidity fluctuations.
Further object of the present invention is to provide a developer that prevents spent carrier, provides sharp and fogless images, and has excellent stability over time.
The above-mentioned object is achieved by a developer, which is preferred embodiment of the present invention, comprising:
toner particles comprising a colorant and a binder resin, said toner particles having a volume-average particle size of 5 to 10 xcexcm;
a first exterior additive comprising a powder selected from the group consisting of hydrophobic titanium oxide powders and hydrophobic alumina powder; and
a second exterior additive comprising a silica powder; said developer satisfying following relationships:
S1 greater than S2
400 less than {square root over ((S12+L +S22+L ))}xe2x89xa61300.
wherein S1 (m2) denotes total specific surface area of the first additive per 1 kg of the toner and S2 (m2) denotes that of the second additive, wherein said total specific surface areas S1 and S2 are represented by following formulas:
S1=A1xc2x7B1
S2=A2xc2x7B2
wherein A1 (m2/g) denotes BET specific surface area of the first additive, A2 (m2/g) denotes that of the second additive, B1 (g) denotes content of the first additive per 1 kg of the toner, and B2 (g) denotes that of the second additive.